Encryption of a compressed video stream

ABSTRACT

The objective of the present application is to allow the reception of a flux of compressed data by the use of powerful algorithms on terminals having small cryptographic capacities.  
     This objective is achieved by an encryption method of a compressed video flux comprising independent data blocks and differential data blocks, consisting in encrypting according to a different level of encryption the independent data blocks and the differential data blocks.

[0001] The present invention concerns a method for encrypting a compressed video flux, particularly allowing to reinforce the level of security and at the same time not penalising the resources during the decryption.

[0002] Video compression algorithms are based on the fact that generally the differences between an image and the following one are small, and that the expression of the differences represents a quantity of information much more reduced than the whole image. It has been observed that from one image to the other a great amount of information does not change and even is found in a slightly different plane.

[0003] This principle is applied in the formats of the MPEG-2, MPEG-3, or Quick Time type.

[0004] According to these algorithms, a first image, called of reference, is transmitted completely and an analysis of the following images is carried out in order to determine and transmit the differences. According to the MPEG norm we distinguish the frames transmitted integrally (I-Frame) and the differential data of the MV type (movement vector) and of the DFD type (difference between the MV model and the real image).

[0005] According to the known solutions, these data are then encrypted according to an algorithm adapted to the desired level of security.

[0006] In order to maintain a compatibility during the transmission and the processing, each group is encrypted by itself, that is, that the attribution of the frames remains visible, only the content is encrypted.

[0007] With the evolution of storage means it is common to transmit encrypted data, representing for example a film, towards the unit of a user.

[0008] Once the file is stored in the unit, a third can have all the necessary time to try to decrypt the data.

[0009] In order to avoid this risk, a first approach consists in increasing the level of security on the file, that is, to use powerful algorithms with long keys.

[0010] Although this technique is satisfying on the level of security, it presents the inconvenient of imposing important resources on the decryption unit.

[0011] The diversification of viewing means progresses towards the use of data by units having small cryptographic capacities. This is the case, for example, of the new mobile telephones that have a view screen. For this type of unit, the use in real time of sophisticated algorithms is not possible without degrading the performance of the unit.

[0012] Thus, the use of blocks of data encrypted by powerful algorithms is incompatible with the use at destination of all types of user units.

[0013] The objective of the present application is then to enable the use of powerful algorithms with terminals that have small cryptographic capacities.

[0014] This objective is achieved by an encryption method of a compressed video flux, comprising independent data blocks and differential data blocks, consisting in encrypting according to a different level of encryption the independent data blocks and the differential data blocks.

[0015] By independent data block we understand information allowing to obtain the decompressed signal without reference to the previous information. It can be for example complete frames (I-Frame).

[0016] By differential data blocks we understand information allowing to obtain the decompressed signal by modification of the previous signal applying this differential information.

[0017] In fact, this solution allows to concentrate the maximum security on the information indispensable for the decompression of the images. According to this method, a first algorithm is applied on the complete frames (I-Frame) of the compressed video signal, and a second algorithm is applied to the differential information of the MV type or of the DFD type.

[0018] This difference can also be done by the use of keys of a different length according to the type of data. Thus, the complete frames will be encrypted by a key of 2048 bits, while the differential information will be encrypted by a key of 128 bits.

[0019] According to an embodiment of the invention, the differential information is not encrypted.

[0020] It should be noted that other sources of information that work on the differential principle can also use this method. This is the case for example of the compressed music according to the MP3 format.

[0021] The present invention will be better understood with the aid of the annexed figures, taken as non-limiting, in which:

[0022]FIG. 1 illustrates the compressed flux before the encryption operation,

[0023]FIG. 2 represents the compressed flux in encrypted form,

[0024]FIG. 3 represents a compressed flux during its transmission.

[0025] In FIG. 1, the compressed flux is represented by a series of frames of the complete type (I) and of differential information (P/C). According to this example, a first complete frame I1, is followed by frames P/C 1-2 allowing to rebuild the successive frames between the complete frame I1 and the frame I2.

[0026] In the same way, the complete frame I2 is followed by successive differential frames P/C 2-3 allowing to reach the complete frame I3.

[0027] This flux is then selectively encrypted according to the type of frame such as illustrated in FIG. 2. In this Figure, we have used a first key k1 of a length of 2048 bits to encrypt the complete frames I1, I2, and I3. A second key k2, for example of 128 bits, has been used for the encryption of the differential frames P/C 1-2 and P/C 2-3.

[0028] The length of the keys k1 and k2 is given here as an indication and could be of any other length.

[0029] According to the invention, the difference of quality of the encryption can be done at the level of the keys or at the level of the used algorithm. Thus, the encryption according to k1 represents for example an algorithm of the IDEA type, and the encryption according to k2 represents an algorithm of the DES type.

[0030] During the diffusion of this flux, the complete frames are sent at first as illustrated by FIG. 3.

[0031] This particularity allows the receiving unit to begin immediately the decryption of the frames that need a long processing. Once these frames have been decrypted, the processing of the differential frames can be done in real time due to the quick execution of the type of algorithm chosen for these frames.

[0032] According to an embodiment of the invention, the level of encryption for the complete frames is different depending on if it is a first frame, such as the frame I1, or the following frames (I2 and I3). In fact, to benefit from the decrypted and decompressed signal, we have to immediately process the first frame and then the frames of differences. This is why the first frame of a series is encrypted with an algorithm of faster decryption than the following complete frames.

[0033] This algorithm can be the same than the one for the differential frames or another algorithm. 

1. A method of encryption of a compressed video flux, comprising independent data blocks (I) and differential data blocks (P/C), characterised in that it consists in encrypting according to a different level of encryption the independent data blocks (I) and the differential data blocks (P/C).
 2. A method of encryption of a compressed video flux according to claim 1, characterised in that the independent data blocks (I) are encrypted by a high level algorithm, while the differential data blocks (P/C) are encrypted by a rapid decryption algorithm.
 3. A method according to claim 1, characterised in that the independent data blocks (I) are encrypted by one or several long keys, while the differential data blocks (P/C) are encrypted by one or several short keys.
 4. A method according to one of the claims 1 to 3, characterised in that during the transmission of the compression signal the independent data blocks (I) are grouped by series, the method consisting in encrypting the first block of this series according to a different level of encryption than that of the following independent blocks. 